Devices and methods for detecting and treating inadequate tissue perfusion

ABSTRACT

Devices and methods for detecting inadequate tissue perfusion by measuring a parameter other than heart rate such as vascular blood pressure, intracardiac blood pressure, vascular blood flow or tissue perfusion, in addition to or as a substitute for heart rate. Such devices and methods improve the accuracy of determining when and to what degree therapy should be administered to treat inadequate tissue perfusion, such as pre-syncope, syncope, or orthostatic hypotension, particularly in the absence of abnormal cardiac function.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 60/454,951, filed Mar. 12, 2003, which isincorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to devices used in the treatmentof inadequate tissue perfusion. In particular, the present inventionrelates to devices and methods for improving the detection and analysisof episodes of inadequate tissue perfusion to enable more effectivetherapy.

In certain disease states, including, but not limited to, pre-syncope,syncope, and orthostatic hypotension, the cardiovascular system does notadequately respond to decreases in intravascular pressure. Lowintravascular pressure results in under-perfusion of body tissues,particularly upper body tissues such as the brain. In a significantnumber of these cases, a demonstrable cardiac arrhythmia is not presentbut the integrated cardiovascular response is inadequate to correct thehypotensive episode.

Unfortunately, devices used to treat patients with this malady, such aspacemakers or infusion pumps, often do not perform adequately. Thesedevices conventionally rely on ECG and/or electrogram as a means toeffect the control of the delivery of a therapy. For example, when thepatient's heart rate, as detected from ECG and/or electrogram, fallsbelow a predetermined level, a pacemaker delivers electrical stimuli tothe heart to increase the heart rate. The efficacy of this approach islimited to pathophysiologic circumstances in which reductions in tissueperfusion occur at the same time as and to the same degree as reductionsin heart rate. If the pathologic drop in tissue perfusion occurs in thepresence of a normal cardiac rhythm and rate, the use of ECG and/orelectrogram is inadequate as means of determining when and to whatdegree therapy should be delivered.

BRIEF SUMMARY OF THE INVENTION

To address this problem, the present invention provides, in exemplarynon-limiting embodiments, devices and methods for detecting inadequatetissue perfusion by measuring a parameter other than heart rate. Forexample, by measuring peripheral vascular blood pressure, intracardiacblood pressure, vascular blood flow or tissue perfusion in addition toor as a substitute for heart rate as measured by ECG or electrogram, thepresent invention improves the accuracy of determining when and to whatdegree therapy should be administered to treat inadequate tissueperfusion. The present invention also provides devices and methods todetect in real time any discrepancy between hemodynamic status andcardiac response, and to then direct interventions appropriately.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow chart illustrating the basic steps involvedin a method of detecting and providing therapy for inadequate tissueperfusion, according to an exemplary non-limiting embodiment of thepresent invention.

FIG. 2 is a schematic illustration of a system including an implantablepressure sensing device and a pacemaker.

FIG. 3 is a schematic illustration of an alternative implantablepressure sensing device.

FIG. 4 is a schematic illustration of a pressure sensing device and apacemaker shown implanted in a patient.

FIG. 5 is a schematic illustration of a flow sensing device that may beused in place of the pressure sensing device.

FIG. 6 is a schematic illustration of a flow sensing device and apacemaker shown implanted in a patient.

FIG. 7 is a schematic illustration of a tissue perfusion monitor thatmay be used in place of the pressure sensing device.

FIG. 8 is a schematic illustration of a tissue perfusion monitor and apacemaker shown implanted in a patient.

FIG. 9 is a schematic illustration of a system including an implantablepacemaker and an implantable pressure sensing device.

FIG. 10 is a schematic illustration of a pressure sensing device with ananchoring electrode shown implanted across a patient's atrial septalwall.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description should be read with reference to thedrawings in which similar elements in different drawings are numberedthe same. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention.

Generic Description of Methods

With reference to FIG. 1, a method 1000 of detecting and providingtherapy for inadequate tissue perfusion is shown schematically. Thoseskilled in the art will recognize that this method 1000 is illustrativeof general concepts according to an exemplary non-limiting embodiment ofthe present invention for detecting and providing therapy for conditionsof inadequate tissue perfusion such as pre-syncope, syncope, andorthostatic hypotension.

In this illustrative method 1000, one or more indicia (X) of inadequatetissue perfusion are measured 1002 to supplement or replace heart rate(HR) measurements 1004 normally taken by ECG or electrogram. The ECG orelectrogram measurement 1004 may be obtained using conventionaltechniques such as utilizing feedback electrodes disposed on a pacemakerimplanted to treat the underlying hypotensive condition. Conventionally,the HR measurements are compared 1006 to a threshold value (Z), that maybe preset, for example, by the treating physician in order to detect anincidence of hypotension. Based on this comparison, which may bemanifested by an algorithm contained in the therapeutic device (e.g.,pacemaker or drug infusion pump), for example, a decision or directive1008 may be made to deliver therapy 1010 if the HR falls below thethreshold value (Z). If the HR exceeds the threshold value (Z), adecision or directive 1008 may be made to not deliver therapy, butrather to continue sampling HR measurements 1004.

However, because hypotension or inadequate tissue perfusion may manifestin a patient without a drop in HR, it is beneficial to measure 1002 oneor more indicia (X) of hypotension or inadequate tissue perfusion. Theone or more indicia (X) may be compared 1012 to a threshold value (Y),and the comparison 1012 may be made by an algorithm executed by thetherapeutic device (e.g., pacemaker or drug infusion pump). Based on thecomparison 1012, a decision or directive 1014 may be made to delivertherapy 1010 if the indicia (X) falls below the threshold value (Y). Ifthe indicia (X) exceeds the threshold value (Y), a decision or directive1014 is made to not deliver therapy, but rather to continue takingmeasurements 1002 of the indicia (X).

The other indicia (X) may comprise, for example, peripheral vascularblood pressure, intracardiac blood pressure, vascular blood flow ortissue perfusion. Any one or a combination of these indicia (X) may bemeasured in addition to or as a substitute for heart rate as measured byECG or electrogram. The indicia may be measured by a separate device orby incorporating measurement capabilities in the therapeutic device. Theinterpretation, analysis and decision making functions may be carriedout by an algorithm executed by suitable electronics in the therapeuticdevice.

The following detailed description starts with a description ofexemplary methods to measure such indicia (X), followed by a descriptionof exemplary measurement devices for measuring the associated indicia(X).

Description of Vascular and Cardiac Pressure Sensing Method

One of the alternative indicia (X) may comprise, without limitation,vascular or cardiac blood pressure. An example of a pressure sensingdevice (PSD) 10 for measuring peripheral vascular blood pressure isdescribed with reference to FIGS. 2, 3 and 4. An example of pressuresensing device 400 for measuring intracardiac blood pressure isdescribed with reference to FIGS. 9 and 10. Alternative pressuremeasuring devices may also be employed, such as an intravascular stentor a vascular cuff with pressure measurement capabilities.

In this method, heart rate data obtained by ECG and/or electrogram maybe simultaneously sensed and interpreted as pressure sensor dataobtained by a pressure measurement device (e.g., device 10 or 400) tomeasure pressure in an artery or within the heart. The therapeuticdevice (e.g., a pacemaker or drug infusion pump) may evaluate thepressure sensed by the pressure measurement device on an ongoing basis,and may create a baseline or reference pressure that is the average ofthe pressures measured over a designated time period (e.g., 30 seconds).If the current pressure has dropped more than a predesignated amountbelow the reference pressure, the therapeutic device begins to deliverstimuli (e.g., electrical pulse series from a pacemaker or a bolus ofdrug from an infusion pump) to the heart to increase heart rate by apredetermined (programmable) amount. If the pressure does not return towithin a predesignated range of the original reference value, thetherapeutic device delivers further stimuli to increase heart rate.During the delivery of therapeutic stimulus, the pressure measurementdevice may periodically sample blood pressure and the therapeutic devicemay re-evaluate the pressure relative to the reference pressure. Ifpressure has returned to within an acceptable range of the referencepressure, the therapeutic device may begin a sequence whereby thestimulus is decreased and heart rate is gradually returned to a normalvalue. If the heart rate reaches a preset upper limit, stimulus deliverymay be terminated, even if the measured pressure is below the acceptabletarget range.

Those skilled in the art will recognize that the reference pressure mayor may not be adjusted for barometric pressure variations. The use of areference pressure without barometric correction (as opposed to acontrol algorithm that employs an absolute pressure) obviates the needto employ a barometric pressure reference for correction of theintravascular or endocardial pressure measurements. This is valuable inthat the need for a barometric pressure monitor adds complexity and costto the system and may also require patient compliance, depending on howthe barometric correction were implemented.

Description of Vascular Flow Sensing Method

One of the alternative indicia (X) may comprise, without limitation,vascular or cardiac blood flow. An example of a flow sensing device(FSD) 210 for measuring peripheral vascular blood flow is described withreference to FIGS. 5 and 6. Alternative devices for measuring blood flowmay also be employed.

With blood flow measurements, the functional control of the stimulusdelivery from the therapeutic device may be the same or similar asdescribed with regard to blood pressure measurements. In other words,the delivery of stimulus may be triggered, maintained and/or shut-offusing pre-programmed thresholds and ranges of blood flow similar to thatwhich has been described previously for blood pressure.

With the flow sensing device, it may be beneficial for noise reductionpurposes that the flow signal be integrated over a programmable numberof complete cardiac cycles (e.g., 2 or 3 cycles). The integrated signal,referred to herein as the current flow value (CFV), may be compared to abaseline value comprising a running average of CFVs occurring over aprogrammed period of time. For example, CFVs that fall within a timeinterval (e.g., 30 second to 10 minute) prior to the current measurementmay be used to create a baseline value of flow. The computed baselinevalue may serve as a reference value and current CFV may be compared tothe reference value in a manner as described previously to detectchanges in flow indicative of a need to modify heart rate with thetherapeutic device.

Description of Tissue Perfusion Method

One of the alternative indicia (X) may comprise, without limitation,tissue perfusion. An example of a tissue perfusion monitor 310 (TPM) isdescribed with reference to FIGS. 7 and 8. Alternative tissue perfusionmeasurement devices may also be utilized.

With tissue perfusion measurements, the functional control of thestimulus delivery from the therapeutic device may be the same or similaras described with regard to blood pressure measurements. In other words,the delivery of stimulus may be triggered, maintained and/or shut-offusing pre-programmed thresholds and ranges of tissue perfusion similarto that which has been described previously for blood pressure.

A reference value for tissue perfusion may be obtained over aprogrammable period of time (e.g., 30 seconds to 10 minutes) to computea running average. The computed running average, which may beperiodically update, may serve as a reference value and currentperfusion measurements may be compared to the reference value in amanner as described previously to detect changes in tissue perfusionindicative of a need to modify heart rate with the therapeutic device.

Description of Vascular Pressure Sensing Device

With reference to FIG. 2, an implantable pressure sensing device (PSD)10 and an implantable therapeutic device (ITD) 60 are shown. By way ofexample, not limitation, the ITD 60 is shown in the form of a pacemaker.The ITD 60 may comprise other therapeutic devices that increase heartrate, such as a drug infusion pump or a pacemaker. The followingdisclosure is given with specific reference to pacemaker, but isunderstood to be equally applicable to other ITDs.

The PSD 10 is connected to the pacemaker 60 by an electrical lead 30.PSD 10 measures blood pressure and generates an electrical pressuresignal which is transmitted in analog or digital form to the pacemaker60 via lead 30. The lead 30 is preferably flexible, and may be similarto conventional pacing leads. A releasable connector 40 may be providedon the pacemaker 60 to facilitate easy connection and disconnection ofthe lead 30. This provides the physician with flexibility duringplacement of the lead 30 and PSD 10 as well as replacement of the PSD 10at a later time should it fail or when the battery depletes.

The pacemaker 60 may otherwise be substantially conventional, with theexception of suitable signal processing electronics to receive andanalyze (e.g., by a suitable algorithm) the pressure signal generated bythe PSD 10. The pacemaker 60 may utilize a conventional endocardial lead70 with a distal endocardial electrode 80 (as shown) to deliver thedesired therapeutic electrical stimulus. Alternatively, subcutaneous(i.e., non-endocardial) electrodes may be used, such as those describedin U.S. Patent Application Publication No. 2002/0107559 to Sanders etal., assigned to Cameron Health, the entire disclosure of which isincorporated herein by reference.

The PSD 10 includes a hermetically sealed housing 12 containing apressure transducer 14 that converts fluidic pressure measurements orsignals into electrical signals. The transducer 14 may be directlycoupled by a plurality of wires 16 to lead 30 which transmits theelectrical signals to the pacemaker 60, which provides the necessarysignal processing and power supply functions. Alternatively, as seen inFIG. 2, the PSD 10 may provide these functions by containing withinhousing 12 an electronics module 13 and battery 15 for signal processingand power functions, respectively. These features are described in moredetail in U.S. Pat. No. 6,033,366, to Brockway et al., the entiredisclosure of which is incorporated herein by reference.

The PSD 10 also includes a pressure transmission catheter 20. The PTC 20has a proximal end connected to the housing 12 and a distal end sizedfor insertion into a vascular lumen. The PTC 20 also includes a lumen influid communication with the pressure transducer contained in thehousing 12. The lumen of the PTC 20 may be filled with a viscous fluid22, with a distally disposed barrier 24 (e.g., gel plug or ePTFEmembrane) that readily transmits pressure signals, but otherwise retainsthe fluid in the lumen of the PTC 20. Further aspects of the PTC 20 aredisclosed in U.S. Pat. No. 4,846,191 to Brockway et al., the entiredisclosure of which is incorporated herein by reference.

A significant benefit of the PTC 20 for measurement of pressure in avascular lumen is that the size of the PTC 20 may be quite small. Forexample, the PTC 20 may be approximately 0.5 mm-1.5 mm diameter, whichis substantially smaller than the 3.5 mm diameter pressure-sensingcatheter used on the Chronicle™ device. In addition to a much smallerdiameter, the portion of the PTC 20 that is inserted into the artery toassure a stable placement and obtain accurate pressure measurements isonly about 5 mm to 10 mm, thus allowing the PTC 20 to be relativelyshort. One benefit of small size is that there is a much lower surfacearea of the sensor exposed to the blood. The smaller the surface area(all other factors such as material properties being equal) the lesserthe risk of thrombo-embolism. A further benefit of smaller size is thatthe risk of hematoma is reduced (a small puncture in the vessel wall ismore likely to seal tightly than is a larger hole). The smaller andlighter PSD 10 is more easily inserted (a small introducer can be usedthat results in significantly less bleeding during insertion and theneed for extended application of pressure to stop bleed afterintroduction is greatly reduced), and is less likely to damage theendothelial surface (because lower mass and size is less likely to causetrauma if it bumps into the vessel wall as a result of blood flow eddiesand changes in patient posture).

The PSD 10 and/or the pacemaker 60 may optionally include ECG electrodesfor measuring heart rate and other electrophysiological parametersassociated with cardiac function. For example, electrodes may beincorporated on the housing of the pacemaker 60, on the electrode lead70 of the pacemaker 60, on the interconnect lead 30 between the PSD 10and pacemaker 60, on the housing 12 of the PSD 10, and/or on the PTC 20of the PSD 10. Such ECG electrodes may be electrically coupled to thesignal processing circuitry of the pacemaker 60.

With reference to FIG. 3, the PSD 10 and the pacemaker 60 are shownimplanted in a patient 100. The pacemaker 60 may be implanted in any ofa number of conventional manners, such as with the lead 70 extendingendocardially through the superior vena cava 112, through the rightatrium 114, with the electrode 80 residing in the right ventricle 116 asshown. Alternatively, the electrode may reside in the right atrium 1 14,the coronary sinus, etc. As mentioned before, non-endocardial electrodeplacement may also be used, such as subcutaneous placement.

The PSD 10 is implanted in the patient 100 with at least the distal endof the PTC 20 disposed in a vascular lumen, such as the subclavianartery 118, while the housing 12 of the PSD 10 remains outside thesubject vascular lumen. The relatively small diameter and short lengthof the PTC 20 has minimal impact on blood flow. Arterial placement ofthe PTC 20 may be preferred over venous placement since the superiorvena cava 112 already contains lead 70, and additional obstructions maycompromise blood flow.

Although the PTC 20 is shown disposed in the subclavian artery 118,those skilled in the art will recognize that other non-endocardial orperipheral vascular sites are also possible, such as the pulmonaryartery, brachial artery or the femoral artery, for example. Furthermore,although the PSD 10 provides significant benefit for detectinghypotension when used to measure pressures in non-endocardial sites(e.g., peripheral artery), the PSD 10 may also be effectively used inthis application for measuring endocardial pressure in any chamber ofthe heart 110. An example of this latter approach is described withreference to FIGS. 9 and 10.

To determine if the patient is experiencing hypotension, the signalprocessing circuitry of the pacemaker 60 evaluates the pressure signalgenerated by the PSD 10, either alone or in combination with an ECGsignal. Signal processing circuitry known to those skilled in the artmay be used to detect hypotension as a trigger for stimulus. A function(e.g., algorithm) for both the pressure and ECG signals may be used toindicate the likelihood that a hypotensive episode requiring stimulus isoccurring in the patient 100.

Description of Vascular Flow Sensing Device

With reference to FIGS. 5 and 6, a flow sensing device (FSD) 200 may beused in place of the PSD 10 described with reference to FIGS. 2, 3 and4. In this alternative embodiment, the FSD 200 measures blood flow rateand generates an electrical flow signal which is transmitted in analogor digital form to the pacemaker 60 via lead 30. The flow measurementsignal is indicative of inadequate tissue perfusion and the need todeliver stimulus with pacemaker 60.

FSD 200 includes a transducer cuffassembly 210 and an electronicsassembly 220. Cuff assembly 210 includes a housing 212 sized and shapedto fit around a blood vessel, such as subclavian artery 118. Cuffhousing 212 may be formed of a flexible polymer or rubber such assilicone rubber. Alternatively, cuff housing 212 may be formed of a morerigid moldable biocompatible polymer, for example, and may available indifferent sizes to accommodate vessels of different diameters. Anexample of a suitable cuff design is disclosed in U.S. PatentApplication Publication No. 2002/0072731 to Doten et al., the entiredisclosure of which is incorporated by reference.

A plurality of transducers 214 are disposed in the housing 212 atdiametrically opposite positions so as to direct ultrasound at thevessel at a 45 degree angle, for example, to facilitate flow measurementwithin the vascular lumen. The transducers 214 may be ultrasonictransducers, for example, and blood flow may be measured by continuouswave Doppler, pulsed Doppler, or transit time techniques, for example.Other flow measurement techniques such as thermal dilution may be usedas well. The transducers 214 of the cuff assembly 210 may be connectedto a separate electronics assembly 220 by lead 218. Electronics assembly220 includes a hermetically sealed housing 222 containing a suitablesignal processing circuit 224 and battery power source 226. An exampleof a suitable transducer arrangement and electronics assembly isdescribed in U.S. Pat. No. 5,865,749 to Doten et al., the entiredisclosure of which is incorporated herein by reference.

It may be beneficial to employ a pulsed Doppler technique to allow forflow to be measured using very low power by positioning a single Dopplerflow crystal on the outer surface of the artery or vein. Such Dopplerflow “cuffs” are available commercially from Crystal Biotech (Hopkinton,Mass.) and by Prof. Craig Hartley (Baylor School of Medicine, Houston,Tex.). Such a cuff could be located on many different arteries and veinslocated under the skin or within the body, but the subclavian artery orvein would be a good choice. Optionally, the Doppler flow crystal may beincorporated into the pacing lead 70, or into a flexible lead designedspecifically for that purpose, such as those commercially available fromMillar Instruments (Houston, Tex.). Such a lead may plug directly intothe header of the pacemaker via a connector similar to an IS1 connector.

Description of Tissue Perfusion Monitor

With reference to FIGS. 7 and 8, a tissue perfusion monitor (TPM) 310may be used in place of the PSD 10 described previously. In thisalternative embodiment, the TPM 310 measures blood perfusion in bodilytissue and generates a blood perfusion signal which is transmitted inanalog or digital form to the pacemaker 60 via lead 30. The degree oftissue perfusion as measured by TPM 310 may be used in a manner similarto how hemodynamic measurements can be used to provide a more effectivetherapy, either alone or in combination with other information such asECG and pressure.

The TPM 310 may utilize, for example, laser Doppler techniques tomeasure blood perfusion in tissue. The laser Doppler flow sensorreflects laser light off bodily tissue and the return signal isindicative of the movement of blood through capillaries contained in thetissue. The sensor may be incorporated into the therapeutic device 60(e.g., implantable housing or electrode lead) or may comprise a separatedevice 310 as shown. The sensor may be located in an area where changesin tissue perfusion would not be induced by everyday activities such aspressure applied to an area of the skin. In order to maintain currentdrain of the sensor at a sufficient low level, it may be beneficial toduty cycle the sensor such that it takes measurements at regularintervals, between which the current drain of the electronics is reducedto a minimum.

As seen in FIG. 7, the TPM 310 includes a hermetically sealed housing312 containing a source of coherent light (e.g., laser) 316 and one ormore photodetectors 318 with associated collecting lenses 314 whichinterface with the tissue to be monitored. For purposes of the clinicalapplications discussed herein, any well vascularized tissue may bemonitored at a convenient in-vivo site such as adjacent the pacemaker 60as shown in FIG. 8. The photodetectors 318 are connected to suitablesignal processing circuitry 322 powered by battery 320. Examples ofsuitable laser Doppler componentry may be found in U.S. Pat. No.6,259,936 to Boggett et al. and European Patent Application No.0282210A1 to Fujii.

A benefit of the TPM 310 is that it does not require insertion into anartery or cardiac chamber. Another benefit is that the TPM 310 may beincorporated as an integral part of the pacemaker 60, with the lensesextending through the housing and the light emitter/detector andelectronics disposed inside the housing, thus eliminating the need foradditional leads. This would have particular benefit for use withsubcutaneously implanted defibrillators, since it is an objective ofsuch devices to eliminate the use of any leads.

Description of Intracardiac Pressure Sensing Device

With reference to FIGS. 9 and 10, a combined pressure sensing andelectrode device (PSED) 400 is shown. The PSED 400 generally includes aPSD 10 as described previously, in addition to an anchoring electrode410, which facilitates both as a stimulus electrode for pacing purposes,and as an anchor to hold the PSD 10 to the atrial septal wall 115, forexample, with the PTC 20 extending across the septal wall 115 and intothe left atrium 117. The PSED 400 in conjunction with the pacemaker 60allows for both the delivery of therapeutic stimulus (e.g., pacing) viaelectrode 410 to the intra-atrial septum 115 and the measurement ofpressure in the left atrium 117 for feedback and triggering purposes,for example.

Those skilled in the art will recognize that the PSED 400 may bepositioned such that the PSD 10 resides inside or outside the heart andthe distal end of the PTC 20 resides in any desired chamber of the heart110. For example, the PTC 20 may be positioned across the leftventricular lateral wall such that the distal end of the PTC 20 isdisposed in the left ventricle and the PSD 10 is mounted to theepicardial surface of the left ventricular lateral wall. As analternative, the PTC 20 may be positioned across the right ventricularlateral wall such that the distal end of the PTC 20 is disposed in theright ventricle and the PSD 10 is mounted to the epicardial surface ofthe right ventricular lateral wall. As a further alternative, the PTC 20may be positioned across the atrial septal wall or the ventricularseptal wall such that the distal end of the PTC 20 is disposed in theleft or right atrium or the left or right ventricle, respectively, withthe PSD mounted to the opposite side of the septal wall.

The PSED 400 may be implanted as shown in FIG. 10 by using conventionalatrial pacing procedure to position the PSED 400 in the right atrium114. The PTC 20 may be disposed across the atrial septal wall 115 byusing trans-septal approach similar to that which is used to deliverelectrophysiology catheters in the left atrium 117. For example, usingfluoroscopic visualization, a guide wire or needle may be used topuncture the septal wall 115, and radiopaque dye may be injected toconfirm complete puncture and placement. A dilator and sheath may beadvanced over the guide wire to access the left atrium 117. The PSED 400may then be advanced through the sheath and/or along the guide wire(with the use of a guide wire lumen on the side of the PSED 400), untilthe PTC 20 extends across the punctured septum 115. The PSED 400 maythen be rotated (for a corkscrew-type anchor) or pushed (for a barb-typeanchor) to secure the PSED 400 to the septal wall 115.

In this alternative embodiment, the PSED 400 measures blood pressure inthe left atrium and generates an electrical pressure signal which istransmitted in analog or digital form to the pacemaker 60 via lead 30.

From the foregoing, it will be apparent to those skilled in the art thatthe present invention provides, in exemplary no-limiting embodiments,implantable devices that measure vascular pressure, vascular blood flow,tissue perfusion, and/or intracardial pressure, and provide feedbackdirectly to a therapeutic device to improve detection and treatment ofinadequate tissue perfusion. Further, those skilled in the art willrecognize that the present invention may be manifested in a variety offorms other than the specific embodiments described and contemplatedherein. Accordingly, departures in form and detail may be made withoutdeparting from the scope and spirit of the present invention asdescribed in the appended claims.

1. A medical method for detecting and treating inadequate tissueperfusion of a patient, comprising: providing a sensor for measuring anintravascular blood parameter; positioning the sensor on a portion ofthe patient's vasculature; measuring the intravascular parameter usingthe sensor; detecting inadequate tissue perfusion based on theintravascular parameter measured by the sensor; delivering a stimulus toincrease tissue perfusion as a function of the measured intravascularparameter.
 2. A medical method as in claim 1, wherein the sensormeasures blood pressure, and wherein the sensor is positioned on a bloodvessel.
 3. A medical method as in claim 2, wherein the sensor includes atransducer and a catheter, wherein the catheter extends through a walland inside a lumen of the blood vessel and the transducer residesoutside the blood vessel.
 4. A medical method as in claim 1, wherein thesensor measures blood flow rate, and wherein the sensor is positioned ona blood vessel.
 5. A medical method as in claim 1, wherein the sensor ispositioned on an artery.
 6. A medical method as in claim 1, wherein thesensor is positioned on an vein.
 7. A medical method for detecting andtreating inadequate tissue perfusion of a patient, comprising: providinga sensor for measuring intracardiac pressure; positioning the sensor inor on the patient's heart; measuring intracardiac pressure of the leftside of the patient's heart using the sensor; detecting inadequatetissue perfusion based on the intracardiac pressure measurement;delivering a stimulus to increase tissue perfusion as a function of theintracardiac pressure measurement.
 8. A medical method as in claim 7,wherein the measured intracardiac pressure comprises left atrialpressure.
 9. A medical method as in claim 7, wherein the measuredintracardiac pressure comprises left ventricular pressure.
 10. A medicalmethod as in claim 7, wherein the sensor is positioned on a chamberwall.
 11. A medical method as in claim 10, wherein the chamber wallcomprises a septal wall.
 12. A medical method as in claim 10, whereinthe chamber wall comprises a free wall.
 13. A medical method as in claim10, wherein the sensor includes a transducer and a catheter, wherein thecatheter extends through the chamber wall into a cardiac chamber and thetransducer resides outside the chamber.
 14. A medical method as in claim13, wherein the sensor is connected to a pacing electrode and the pacingelectrode contacts the chamber wall.
 15. A medical method for detectingand treating inadequate tissue perfusion of a patient, comprising:providing a sensor for measuring tissue perfusion; providing atherapeutic device for delivering a stimulus to increase tissueperfusion; positioning the sensor in the patient remote from thetherapeutic device; measuring tissue perfusion using the sensor;detecting inadequate tissue perfusion based on the tissue perfusionmeasurement; and delivering a stimulus to increase tissue perfusion as afunction of the tissue perfusion measurement.
 16. A medical method as inclaim 15, wherein the sensor is positioned adjacent vascularized tissueand measures blood flow in the vascularized tissue.
 17. A medical methodas in claim 16, wherein the sensor measures blood flow in capillaries inthe vascularized tissue.
 18. A medical method for treating a patient,comprising: detecting heart rate as an indicator of inadequate tissueperfusion; detecting at least one other indicia of inadequate tissueperfusion; delivering a stimulus to increase tissue perfusion as afunction of both heart rate and the at least one other indicia.
 19. Amedical method as in claim 18, further comprising providing atherapeutic device for delivering the stimulus to increase tissueperfusion.
 20. A medical method as in claim 19, wherein the step ofdelivering the stimulus comprises delivering a stimulus to increaseheart rate.
 21. A medical method as in claim 20, wherein the step ofproviding a therapeutic device comprises providing a pacemaker, andwherein the step of delivering the stimulus to increase heart ratecomprises delivering electrical impulses to the patient's heart.
 22. Amedical method as in claim 20, wherein the step of providing atherapeutic device comprises providing an infusion pump, and wherein thestep of delivering the stimulus to increase heart rate comprisesdelivering a bolus of a drug.
 23. A medical method as in claim 20,wherein the step of detecting at least one other indicia of inadequatetissue perfusion comprises detecting blood pressure.
 24. A medicalmethod as in claim 23, wherein the step of detecting blood pressurecomprises detecting vascular blood pressure.
 25. A medical method as inclaim 23, wherein the step of detecting blood pressure comprisesdetecting intracardiac blood pressure.
 26. A medical method as in claim20, wherein the step of detecting at least one other indicia ofinadequate tissue perfusion comprises detecting blood flow.
 27. Amedical method as in claim 26, wherein the step of detecting blood flowcomprises detecting vascular blood flow.
 28. A medical method as inclaim 20, wherein the step of detecting at least one other indicia ofinadequate tissue perfusion comprises detecting blood perfusion intissue.
 29. A medical method as in claim 28, wherein the step ofdetecting blood perfusion in tissue comprises detecting blood perfusionin tissue in the patient's upper body.
 30. A medical method as in claim28, wherein the step of detecting blood perfusion in tissue comprisesdetecting blood perfusion in tissue in the patient's chest.
 31. Amedical method as in claim 28, wherein the step of detecting bloodperfusion in tissue comprises detecting blood perfusion in tissue in thepatient's head or neck.
 32. A medical method, comprising: providing animplantable therapeutic device (ITD) configured to deliver a stimulus toincrease heart rate; providing an implantable pressure sensing device(PSD) including a hermetically sealed housing, a pressure transducerdisposed in the housing, a pressure transmission catheter (PTC) having aproximal end, a distal end, and a lumen extending therethrough, with theproximal end of the PTC connected to the housing and the lumen of thePTC in fluid communication with the pressure transducer; implanting theITD in a patient; implanting the PSD in the patient such that the distalend of the PTC resides in a vascular lumen and the housing remainsoutside the vascular lumen; connecting the PSD to the ITD via anelectrical lead; and operating the ITD to deliver the stimulus toincrease heart rate in response to a drop in blood pressure as measuredby the PSD.
 33. A method as in claim 32, wherein the pressure transducerof the PSD converts a pressure signal to an electrical signal, andwherein the ITD includes a signal processor which evaluates theelectrical signal for hypotension.
 34. A method as in claim 33, whereinthe lumen of the PTC is filled with a fluid and a barrier is disposed ina distal end of the PTC lumen to contain the fluid while permittingpressure to be transferred therethrough.
 35. A method as in claim 32,wherein the ITD delivers an electrical stimulus.
 36. A method as inclaim 32, wherein the ITD delivers a pharmacological stimulus.